Add showfilename configuration option

[job]

When having a few instances of vi open next to each other (using a window manager), pressing control-G all the time to understand 'what file is in what window' is tedious. Instead, offer a configurable option (default off) to display the file name in the lower left corner.

[job]

A similar patch was accepted into OpenBSD's vi implementation https://github.com/openbsd/src/commit/211058dfe1741937a159d60824c94ca470580b2a

[lichray]

/[...]/vi/vs_refresh.c:798:34: error: use of undeclared identifier 'O_SHOWFILENAME'
        if (IS_SPLIT(sp) || O_ISSET(sp, O_SHOWFILENAME)) {
                                        ^

[job]

@lichray whoops! Better now?

[lichray]

Works great. Thank you!