squidclient: Improve shell-escape support in -H option

squidclient: Improve shell-escape support in -H option

The -H parameter takes a string with some limited shell-escaped
characters. Currently just \n was expanded to the CRLF sequence.
Other shell escaped characters were left untouched.

However, to properly test headers containing weird CR, LF and HTAB
positioning it needs to be able to receive these special characters
individually and thus unescape them.

Add a new function similar to perform shell unescape with special
characters \\, \r, \n, and \t. All other characters are un-escaped
to themselves and a warning printed at verbosity level 1.