BUILD: makefile: bump the default minimum linux version to 4.17

As explained during the 3.3-dev7 announcement below:
  https://www.mail-archive.com/[email protected]/msg46073.html

no regularly maintained distro supports a kernel older than 4.18 anymore,
and KTLS is supported since 4.17. So it's about the right moment to bump
the default minimum kernel version supported by glibc and musl to
automatically cover new features. The linux-glibc-legacy target still
supports 2.6.28 and above.

Author: wtarreau

INSTALL

@@ -559,9 +559,9 @@ It goes into more details with the main options.
 To build haproxy, you have to choose your target OS amongst the following ones
 and assign it to the TARGET variable :
 
-  - linux-glibc         for Linux kernel 2.6.28 and above
+  - linux-glibc         for Linux kernel 4.17 and above
   - linux-glibc-legacy  for Linux kernel 2.6.28 and above without new features
-  - linux-musl          for Linux kernel 2.6.28 and above with musl libc
+  - linux-musl          for Linux kernel 4.17 and above with musl libc
   - solaris             for Solaris 10 and above
   - freebsd             for FreeBSD 10 and above
   - dragonfly           for DragonFlyBSD 4.3 and above

Makefile

@@ -379,13 +379,13 @@ ifeq ($(TARGET),haiku)
   set_target_defaults = $(call default_opts,USE_POLL USE_TPROXY USE_OBSOLETE_LINKER)
 endif
 
-# For linux >= 2.6.28 and glibc
+# For linux >= 4.17 and glibc
 ifeq ($(TARGET),linux-glibc)
   set_target_defaults = $(call default_opts, \
     USE_POLL USE_TPROXY USE_LIBCRYPT USE_DL USE_RT USE_CRYPT_H USE_NETFILTER  \
     USE_CPU_AFFINITY USE_THREAD USE_EPOLL USE_LINUX_TPROXY USE_LINUX_CAP      \
     USE_ACCEPT4 USE_LINUX_SPLICE USE_PRCTL USE_THREAD_DUMP USE_NS USE_TFO     \
-    USE_GETADDRINFO USE_BACKTRACE USE_SHM_OPEN)
+    USE_GETADDRINFO USE_BACKTRACE USE_SHM_OPEN USE_KTLS)
   INSTALL = install -v
 endif
 
@@ -398,13 +398,13 @@ ifeq ($(TARGET),linux-glibc-legacy)
   INSTALL = install -v
 endif
 
-# For linux >= 2.6.28 and musl
+# For linux >= 4.17 and musl
 ifeq ($(TARGET),linux-musl)
   set_target_defaults = $(call default_opts, \
     USE_POLL USE_TPROXY USE_LIBCRYPT USE_DL USE_RT USE_CRYPT_H USE_NETFILTER  \
     USE_CPU_AFFINITY USE_THREAD USE_EPOLL USE_LINUX_TPROXY USE_LINUX_CAP      \
     USE_ACCEPT4 USE_LINUX_SPLICE USE_PRCTL USE_THREAD_DUMP USE_NS USE_TFO     \
-    USE_GETADDRINFO USE_BACKTRACE USE_SHM_OPEN)
+    USE_GETADDRINFO USE_BACKTRACE USE_SHM_OPEN USE_KTLS)
   INSTALL = install -v
 endif
 

doc/configuration.txt

@@ -16801,8 +16801,8 @@ interface <interface>
 
 ktls <on|off> [ EXPERIMENTAL ]
   Enables or disables ktls for those sockets. If enabled, kTLS will be used
-  if the kernel supports it and the cipher is compatible.
-  This is only available on linux.
+  if the kernel supports it and the cipher is compatible. This is only
+  available on Linux kernel 4.17 and above.
 
 label <label>
   Sets an optional label for these sockets. It could be used group sockets by