maint: remove form feed characters from sources

maint: remove form feed characters from sources

These are no longer useful and can impact viewing
patches in some mail programs for example.

* src/cksum.c: Remove ^L characters.
* src/comm.c: Likewise.
* src/kill.c Likewise.
* src/ln.c Likewise.
* src/nl.c Likewise.
* src/pr.c Likewise.
* src/ptx.c Likewise.
* src/split.c Likewise.
* src/tr.c Likewise.
* src/tsort.c Likewise.
* src/uniq.c Likewise.
* src/wc.c Likewise.
* cfg.mk (sc_prohibit-form-feed): A new syntax check.