Turn MALLOC_PRODUCTION into a regular src.conf(5) option

For historical reasons, defining MALLOC_PRODUCTION in /etc/make.conf has
been used to turn off potentially expensive debug checks and statistics
gathering in the implementation of malloc(3).

It seems more consistent to turn this into a regular src.conf(5) option,
e.g. WITH_MALLOC_PRODUCTION / WITHOUT_MALLOC_PRODUCTION. This can then
be toggled similar to any other source build option, and turned on or
off by default for e.g. stable branches.

Reviewed by: imp, manpages
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D26337