Only print the warning that gopclntab can not be read for the first
image (i.e. the executable file), also change the returned when neither
DWARF nor gopclntab are found to preserve the DWARF error.
We used to parse the .gopclntab section but removed support in favor of
simply using DWARF debug information, due to lack of C symbols among
other reasons. This makes it impossible to debug stripped binaries,
which some distrubutions ship by default.
Add back in basic support for .gopclntab which survives if the binary
is stripped, allowing for rudimentary debugging such as basic
program navigation, tracing, etc...
