Fix 'info proc cmdline' for native FreeBSD processes.

Fix 'info proc cmdline' for native FreeBSD processes.

The kern.proc.args.<pid> sysctl returns the argv array as a packed
array of arguments, each null terminated.  To construct a complete
command line, the arguments must be joined with spaces by converting
the intermediate nul characters to spaces.  Previously only the first
argument was shown in cmdline output.  Now, all arguments are shown.

gdb/ChangeLog:

	* fbsd-nat.c (fbsd_fetch_cmdline): Join arguments with spaces.