init_socketpath() was building a sprintf() format string intended to be used by /DETACH to create the socket
file name. This included the actual socket path, plus a %d for the port, plus the sanitised tty name and
hostname.
Only one caller needed all this though - the /DETACH command - and the other callers (in scr-bx.c) just
wanted to truncate it to the actual socket path. The format string also wasn't safe - if the home directory
path, hostname or ttyname contained % characters these werent being escaped.
It simplifies things to have init_socketpath() just return the actual socket path, after creating the 'screens'
directory if necessary. This lets the code in scr-bx.c use it as-is, and removes the need for the global
socket_path variable. The code to include the sanitised tty name and hostname in the socket file name can
be moved to the create_ipc_socket() function.
There's no need to check access() for the socket path before trying to create it - just call mkdir() regardless,
since it will fail if the path already exists, which is fine.
This commit also adds error handling to the create_ipc_socket() function for the case where creation of the
socket file fails, and switches the chmod() and chown() for the opened file to the more appropriate fchmod()
and fchown().
492d5658d1