sslutil: abort when ssl module is needed but not found
It is apparently possible to compile Python without SSL support or leave it out
when installing precompiled binaries.
Mercurial on such Pythons would crash if the user tried to use https. Now it
will be reported as "abort: Python SSL support not found" instead.
patch: correctly handle non-tabular Subject: line
The line content of continued Subject: lines was yet joined via
str.replace('\n\t', ' '), which does not handle continuation via
spaces. So expan the regular expression instead to
handle all allowed forms of mail header line continuation.