Fix regular /path/to/directory sysroots and target reported dll paths with drive specs.

I tried debugging a remote Windows program on Linux host, and pointed the
sysroot to "/some/path/" rather than "remote:", and I found GDB couldn't
find the dlls in the sysroot.  If the dll name is
"C:/Windows/system32/ntdll.dll", I end up with the sysroot+in_pathname
concatenated this way:

 (top-gdb) p temp_pathname
 $1 = 0x228b690 "/some/pathC:/Windows/system32/ntdll.dll"
                          ^^

That is, a directory separator is missing.  This is a regression.

The problem is that solib_find decides that since the target path has
a drive spec, a separator is not necessary, which is clearly wrong in
this case.  That check was added in
<https://sourceware.org/ml/gdb-patches/2013-06/msg00028.html>, to
handle the case of sysroot being "remote:".  This patch fixes that
original issue in a different way.  Instead of checking whether the
path has a drive spec, check whether the sysroot is "remote:".  The
patch adds a table that helps visualize the cases that need a
separator.  I also confirmed the original issue is still handled as
expected.  That is, that "set sysroot remote:" still does the right
thing.

remote_filename_p returns true if the filename is prefixed with
"remote:".  In this case, we need to check whether the filename is
exactly "remote:".  I thought of different ways or either changing
remote_filename_p or adding another convenience function to remote.c
to avoid exposing the "remote:" prefix out of remote.c.  But all
attempts turned out adding lot of over needless complication.  So the
patch just exposes the prefix behind a new macro, which allows using a
straighforward strcmp.

gdb/
2013-09-27  Pedro Alves  <palves@redhat.com>

	* remote.h (REMOTE_SYSROOT_PREFIX): New define.
	(remote_filename_p): Add comment.
	* remote.c (remote_filename_p): Adjust to use
	REMOTE_SYSROOT_PREFIX.
	* solib.c (solib_find): When deciding whether we need to add a
	directory separator, check whether the sysroot is "remote:"
	instead of checking whether the patch has a drive spec.  Add
	comments.