Unify shell-finding logic

Unify shell-finding logic

I noticed several places in gdb that were using getenv("SHELL") and
then falling back to "/bin/sh" if it returned NULL.  This unifies
these into a single function.

gdb/ChangeLog
2018-10-27  Tom Tromey  <tom@tromey.com>

	* procfs.c (procfs_target::create_inferior): Use get_shell.
	* cli/cli-cmds.c (shell_escape): Use get_shell.
	* windows-nat.c (windows_nat_target::create_inferior): Use
	get_shell.
	* common/pathstuff.c (get_shell): New function.
	* nat/fork-inferior.c (SHELL_FILE, get_startup_shell): Remove.
	(fork_inferior): Use get_shell.
	* common/pathstuff.h (get_shell): Declare.