Fix up prototypes of basename(3) and dirname(3) to comply to POSIX.

POSIX allows these functions to be implemented in a way that the
resulting string is stored in the input buffer. Though some may find
this annoying, this has the advantage that it makes it possible to
implement this function in a thread-safe way. It also means that they
can be implemented in a way that they work for paths of arbitrary
length, as the output string of these functions is never longer than
max(1, len(input)).

Portable code already needs to be written with this in mind, so in my
opinion it makes very little sense to allow the existing behaviour.
Prevent the base system from falling back to this by switching over to
POSIX prototypes.

I'm not going to bump the __FreeBSD_version for this. The reason is that
it's possible to account for this change in a portable way, without
depending on a specific version of FreeBSD. An exp-run was done some
time ago. As far as I know, all regressions as a result of this have
already been fixed.

I'll give this change some time to settle. In the long run I want to
replace our copies by ones that are thread-safe and don't depend on
PATH_MAX/MAXPATHLEN.

include/libgen.h

@@ -34,9 +34,9 @@
 #include <sys/cdefs.h>
 
 __BEGIN_DECLS
-char	*basename(const char *);
+char	*basename(char *);
 char	*basename_r(const char *, char *);
-char	*dirname(const char *);
+char	*dirname(char *);
 __END_DECLS
 
 #endif /* !_LIBGEN_H_ */

lib/libc/gen/basename.3

@@ -16,7 +16,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd March 31, 2010
+.Dd July 28, 2016
 .Dt BASENAME 3
 .Os
 .Sh NAME
@@ -25,7 +25,7 @@
 .Sh SYNOPSIS
 .In libgen.h
 .Ft char *
-.Fn basename "const char *path"
+.Fn basename "char *path"
 .Ft char *
 .Fn basename_r "const char *path" "char *bname"
 .Sh DESCRIPTION

lib/libc/gen/basename.c

@@ -66,7 +66,7 @@ basename_r(const char *path, char *bname)
 }
 
 char *
-basename(const char *path)
+basename(char *path)
 {
 	static char *bname = NULL;
 

lib/libc/gen/dirname.3

@@ -16,7 +16,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd October 12, 2006
+.Dd July 28, 2016
 .Dt DIRNAME 3
 .Os
 .Sh NAME
@@ -25,7 +25,7 @@
 .Sh SYNOPSIS
 .In libgen.h
 .Ft char *
-.Fn dirname "const char *path"
+.Fn dirname "char *path"
 .Sh DESCRIPTION
 The
 .Fn dirname

lib/libc/gen/dirname.c

@@ -26,7 +26,7 @@ __FBSDID("$FreeBSD$");
 #include <sys/param.h>
 
 char *
-dirname(const char *path)
+dirname(char *path)
 {
 	static char *dname = NULL;
 	size_t len;