New features

• Add embedding support with -w option. This option can be used to xembed dmenu into an application. This is useful in particular for surf.
• config.h: add config option for word delimiters.

Noteworthy fixes

• die() on calloc failure.
• Sync new drw from libsl and minor fixes.
• arg.h: fixed argv checks order.
• Regression fix: Do not crash on e.g. dmenu < /dev/null
• Shut up glibc about _BSD_SOURCE being deprecated.
• Xinerama: correct variable declarations in preprocessor conditional.
• Small man page improvements.