Define _CS_PATH on the BSDs

According to 
https://man.netbsd.org/confstr.3

_CS_PATH is obsoleted by sysctl (which has a USER_CS_PATH equivalent),
but Linux doesn't have that.

So the simplest thing for applications is to use _CS_PATH which is
part of POSIX. Define, matching the <unistd.h> header.

We could maybe share this definition in src/unix/bsd/netbsdlike/mod.rs,
but I saw that existing definitions are not shared either, so I'm
not sure.

	$ grep src/unix/bsd/netbsdlike -e _PC_LINK_MAX
	src/unix/bsd/netbsdlike/netbsd/mod.rs:1599:11:pub const _PC_LINK_MAX: c_int = 1;
	src/unix/bsd/netbsdlike/openbsd/mod.rs:1193:11:pub const _PC_LINK_MAX: c_int = 1;


Originally reported in fish-shell/fish-shell#11892

Author: krobelus

libc-test/semver/dragonfly.txt

@@ -1128,6 +1128,7 @@ WTRAPPED
 XUCRED_VERSION
 YESEXPR
 YESSTR
+_CS_PATH
 _IOFBF
 _IOLBF
 _IONBF

libc-test/semver/freebsd.txt

@@ -1715,6 +1715,7 @@ XUCRED_VERSION
 XU_NGROUPS
 YESEXPR
 YESSTR
+_CS_PATH
 _IOFBF
 _IOLBF
 _IONBF

libc-test/semver/netbsd.txt

@@ -1127,6 +1127,7 @@ XATTR_CREATE
 XATTR_REPLACE
 YESEXPR
 YESSTR
+_CS_PATH
 _IO
 _IOC
 _IOFBF

libc-test/semver/openbsd.txt

@@ -917,6 +917,7 @@ WSTOPPED
 WTRAPPED
 YESEXPR
 YESSTR
+_CS_PATH
 _IO
 _IOC
 _IOFBF

src/unix/bsd/freebsdlike/dragonfly/mod.rs

@@ -1361,6 +1361,8 @@ pub const VCHECKPT: usize = 19;
 pub const _PC_2_SYMLINKS: c_int = 22;
 pub const _PC_TIMESTAMP_RESOLUTION: c_int = 23;
 
+pub const _CS_PATH: c_int = 1;
+
 pub const _SC_V7_ILP32_OFF32: c_int = 122;
 pub const _SC_V7_ILP32_OFFBIG: c_int = 123;
 pub const _SC_V7_LP64_OFF64: c_int = 124;

src/unix/bsd/freebsdlike/mod.rs

@@ -1197,6 +1197,8 @@ pub const _SC_RAW_SOCKETS: c_int = 119;
 pub const _SC_SYMLOOP_MAX: c_int = 120;
 pub const _SC_PHYS_PAGES: c_int = 121;
 
+pub const _CS_PATH: c_int = 1;
+
 pub const PTHREAD_MUTEX_INITIALIZER: pthread_mutex_t = ptr::null_mut();
 pub const PTHREAD_COND_INITIALIZER: pthread_cond_t = ptr::null_mut();
 pub const PTHREAD_RWLOCK_INITIALIZER: pthread_rwlock_t = ptr::null_mut();

src/unix/bsd/netbsdlike/netbsd/mod.rs

@@ -1612,6 +1612,8 @@ pub const _PC_2_SYMLINKS: c_int = 13;
 pub const _PC_ACL_EXTENDED: c_int = 14;
 pub const _PC_MIN_HOLE_SIZE: c_int = 15;
 
+pub const _CS_PATH: c_int = 1;
+
 pub const _SC_SYNCHRONIZED_IO: c_int = 31;
 pub const _SC_IOV_MAX: c_int = 32;
 pub const _SC_MAPPED_FILES: c_int = 33;

src/unix/bsd/netbsdlike/openbsd/mod.rs

@@ -1212,6 +1212,8 @@ pub const _PC_SYMLINK_MAX: c_int = 19;
 pub const _PC_SYNC_IO: c_int = 20;
 pub const _PC_TIMESTAMP_RESOLUTION: c_int = 21;
 
+pub const _CS_PATH: c_int = 1;
+
 pub const _SC_CLK_TCK: c_int = 3;
 pub const _SC_SEM_NSEMS_MAX: c_int = 31;
 pub const _SC_SEM_VALUE_MAX: c_int = 32;