Minor cleanup after adding more warnings to CFLAGS

I had some time and wanted to see how mutt compiles with more warnings
enabled, i.e. CFLAGS='-Wall -Wcast-qual -Werror
-Wimplicit-function-declaration -Wimplicit-int -Wnull-dereference
-Wold-style-definition -Wredundant-decls -Wstrict-prototypes
-Wstring-compare -Wtype-limits -Wuninitialized -Wunused'

Turns out there's a lot the compiler complains about, especially when it
comes to some const usage. The following patches are a result of this
experiment and are far from a complete fix.

This series depends on the proposed reformatting from Kevin. It's
untested besides compilation and CI.