Use ihex_set_error() in ihex_parse.c.

Align the definition of the IHEX_SET_ERROR*() macros in ihex_parse.c
with the other source files.  Use the common function ihex_set_error()
instead of duplicating the code.
Avoid the variable name "errno".

According to ISO C, the standard "errno" variable may be implemented
as a macro.  This causes conflicts and compiler warnings (e.g. on
MinGW) when used as a function argument or variable name.

To be safe, rename all errno parameters to errnum.  This fixes some
warnings on MinGW.
Merge pull request #26 from acolomb/fix-errno

Fix compiler warnings resulting from invalid "errno" variable name