sip: avoid unneeded calls to strlen

std::string includes the string size.
Use that size to initialize pj_str_t instead of calling strlen.

Also make constexpr some static strings

Change-Id: Icc1cdd16ad17aa03c16f44aef10cbe949e16ff0b
Reviewed-by: Sébastien Blin <sebastien.blin@savoirfairelinux.com>