mail/exim: update to 4.73

Most notably, this version fixes local exim -> root escalation,
CVE-2010-4345.

Port had also gained configurable knob for disabling -D option
and make variables TRUSTED_CONFIG_LIST and WHITELIST_D_MACROS
to fine tune the behaviour of options -C and -D.

New items are documented at

ftp://exim.inode.at/exim/ChangeLogs/NewStuff-4.73

Changelog is available at

ftp://exim.inode.at/exim/ChangeLogs/ChangeLog-4.73

Security: e4fcf020-0447-11e0-becc-0022156e8794 / CVE-2010-4345
PR: 152963 [1], 153711 [2]
Submitted by: Alexander Wittig <alexander@wittig.name> [1]
Approved by: garga (mentor)