[PATCH] SBuf conversion of HttpRequestMethod

Add mk-sbuf-strings.awk script to produce a global array of SBuf for the
string objects of registered methods. This can be re-used to convert
other enum name arrays as we go forward.

Use an SBuf to store unknown method names "image". Including a few
string comparison upgrades.

Removes use of RequestMethodStr() wrapper in debugs to reduce the amount
of overheads calling c_str() when debug printing method strings.

Amos