dmenu: update to 5.1.

code-style: rm newline (oops)
revert using strcasestr and use a more optimized portable version
follow-up fix: add -D_GNU_SOURCE for strcasestr for some systems
improve performance of case-insensitive matching
Revert "Improve speed of drw_text when provided with large strings"
add support for more keypad keys
Improve speed of drw_text when provided with large strings
remove always true condition in if statement

(wiz)