test command fails at makeself-header.sh:170 when md5sum is not available

Hi!

Just a small fix, it happens that Solaris machines do no have md5sum out of the box, which causes the script to fail with:

+ md5=
+ test = 00000000000000000000000000000000
.sh: line 170: test: =: unary operator expected

I've also took the opportunity and went through makeself-header.sh to ensure all the string vars are escaped. Plus I made the comparisons consistent, part of the places where using x$var = xval, other "$var" = val, so I changed them to the latter.