Fix running long external cmds in term multiplexer

Invoked shell in current directory because of trailing backward
slash that accidentally escaped the following argument which is
command to run itself.  This lead to command being treated as part
of title.
Skip leading spaces in terminal multiplexers title
Make title buffer larger
Do not use fixed-size buffers for term mux titles
Remove that safety spaces added before

No need for this "hackish solution" anymore.
Do not use fixed-size buffers for term mux cmds
Extract running.c:set_pwd_in_screen()

Changes $PWD in running GNU/screen session to the specified path.
Needed for symlink directories and sshfs mounts.
Do not use fixed-size buffers for shell commands

It seems that such approach causes troubles that are not covered
by convenience it gives.
Remove max command args length constant

It became useless.
Merge branch 'fix-term-mux-long-title-run'

Fix running external commands with long titles inside terminal
multiplexers.  It caused shell to be invoked in current directory.