Fix fr_utf8_strchar() and tests

allowed characters shouldn't be escaped in the test.

the function returns const

it checks for fr_utf8_char() of str, not char the second time
around
Add libcap-dev dependency for travis and debian packages
Link to libcap to determine process capabilities

This lets us know whether the debugger attach check is failing because
we don't have CAP_SYS_PTRACE or because there really is another
debugger attached already.