strutil: Add function to parse floating point numbers while ignoring the locale.

strutil: Add function to parse floating point numbers while ignoring the locale.

Most of the supported gear uses the ANSI C locale for communication, and if the
client sets up an incompatible locale parsing would fail.

This function ignores the client's locale and parses floating point numbers
using the ANSI C locale. This function should be always used when parsing
floating point numbers coming from the instrument.