gets.3, mbtowc.3, printf.3, puts.3, scanf.3, stpcpy.3, stpncpy.3, strcat.3, strlen...

gets.3, mbtowc.3, printf.3, puts.3, scanf.3, stpcpy.3, stpncpy.3, strcat.3, strlen.3, strnlen.3, strsep.3, strstr.3, strxfrm.3, utmp.5: Global fix: consistent use of "null byte"

Bring more consistency to the discussion of
"[terminating] null byte"
by writing (at least in the initial use in a page)
"[terminating] null byte ('\0')".

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>