Change reentr.pl names to be legal

This continues the process started in Perl#23592 to change names with
leading underscores to be legal C.  See that p.r. or
4bb3572 for extensive discussion.

This commit simply moves the leading underscore to be trailing
Some of these may actually be legal, and don't need to be moved, as
leading underscores are only illegal in symbols with file scope.  But it
was easier to just do a global substitute, and does no harm to move them
all to be trailing.

Author: khwilliamson

pp.c

@@ -4061,15 +4061,15 @@ PP(pp_crypt)
     }
 #  ifdef USE_ITHREADS
 #    ifdef HAS_CRYPT_R
-    if (!PL_reentrant_buffer->_crypt_struct_buffer) {
+    if (!PL_reentrant_buffer->crypt_struct_buffer_) {
       /* This should be threadsafe because in ithreads there is only
        * one thread per interpreter.  If this would not be true,
        * we would need a mutex to protect this malloc. */
-        PL_reentrant_buffer->_crypt_struct_buffer =
+        PL_reentrant_buffer->crypt_struct_buffer_ =
           (struct crypt_data *)safemalloc(sizeof(struct crypt_data));
 #      if defined(__GLIBC__) || defined(__EMX__)
-        if (PL_reentrant_buffer->_crypt_struct_buffer) {
-            PL_reentrant_buffer->_crypt_struct_buffer->initialized = 0;
+        if (PL_reentrant_buffer->crypt_struct_buffer_) {
+            PL_reentrant_buffer->crypt_struct_buffer_->initialized = 0;
         }
 #      endif
     }

pp_sys.c

@@ -5291,7 +5291,7 @@ PP_wrapped(pp_ghostent,
         if (!hent) {
 #ifdef USE_REENTRANT_API
 #   ifdef USE_GETHOSTENT_ERRNO
-            h_errno = PL_reentrant_buffer->_gethostent_errno;
+            h_errno = PL_reentrant_buffer->gethostent_errno_;
 #   endif
 #endif
             STATUS_UNIX_SET(h_errno);
@@ -5382,7 +5382,7 @@ PP_wrapped(pp_gnetent,
         if (!nent) {
 #ifdef USE_REENTRANT_API
 #   ifdef USE_GETNETENT_ERRNO
-             h_errno = PL_reentrant_buffer->_getnetent_errno;
+             h_errno = PL_reentrant_buffer->getnetent_errno_;
 #   endif
 #endif
             STATUS_UNIX_SET(h_errno);
@@ -5735,7 +5735,7 @@ PP_wrapped(pp_gpwent,
 #   if defined(__CYGWIN__) && defined(USE_REENTRANT_API)
     /* Cygwin 1.5.3-1 has buggy getpwnam_r() and getpwuid_r():
      * the pw_comment is left uninitialized. */
-    PL_reentrant_buffer->_pwent_struct.pw_comment = NULL;
+    PL_reentrant_buffer->pwent_struct_.pw_comment = NULL;
 #   endif
 
     switch (which) {