hg-fast-export: add support for -U <addr> option
Instead of always using 'devnull@localhost' as the email address
when it is unknown, allow the address to use when the email is unknown
to be specified by the new -U <addr> option.
The default if -U <addr> is not used remains 'devnull@localhost'.
The specified address may be surrounded by optional whitespace and/or
optional '<' and '>'. The only thing that will cause it to be rejected
is if it's empty (after stripping surrounding "crud" characters and
removing internal '<', '>' and "\n" characters).