Rename getfuncargvalue to getfixturevalue

[tomviner]

getfuncargvalue will be deprecated but will still be supported for the moment.

As @hpk42 mentioned this morning, this is independent of replacing / removing this feature.

[tomviner]

Fixed by #1626 Please close @nicoddemus

(I'll do the magic commit message that closes issues next time)

[RonnyPfannschmidt]

thanks