gitweb: safely output binary files for 'blob_plain' action

gitweb tries now to output correct Content-Type header for
'blob_plain' action; for now text/plain for text files,
appropriate image MIME type for *.png, *.gif and *.jpg/*.jpeg files,
and application/octet-stream for other binary files.

Introduced new configuration variables: $default_blob_plain_mimetype
and $default_text_plain_charset (only 'utf-8' is guaranteed to work
for the latter).

binmode changed to ':raw' in git_blob_plain for output of non-text files.

Signed-off-by: Junio C Hamano <redacted>