Introduce an extended syntax for audio positions

Introduce a new lsx_parseposition function (similar to lsx_parsesamples)
that makes a trim-like syntax for specifying audio positions available to
all effects; it is possible to give a position relative to the start of
audio ("=2:00"), to the end of audio ("-1:30"), or to a previous position
("+1:00").

The original lsx_parsesamples function is replaced by a wrapper for an
internal parsesamples function that allows a start value and an initial
combine mode (add/subtract) to be passed in, which lsx_parseposition
needs.

This commit only introduces the function and common documentation. No
effects are using lsx_parseposition yet.
delay: use lsx_parseposition

Use the new lsx_parseposition function for the delay effect and update
the documentation accordingly.
trim: use lsx_parseposition

Use the new lsx_parseposition function for the trim effect, removing much
of the dedicated parsing as it is now handled by lsx_parseposition. Update
documentation.
fade: use lsx_parseposition

Use the new lsx_parseposition function for the fade effect and update
the documentation accordingly.
spectrogram -S: use lsx_parseposition

Use the new lsx_parseposition function for the -S option to the
spectrogram effect and update the documentation accordingly.
bend: use lsx_parseposition

Use the new lsx_parseposition function for the bend effect and update
the documentation accordingly.
splice: use lsx_parseposition

Use the new lsx_parseposition function for the splice effect and
update the documentation accordingly.
pad: use lsx_parseposition

Use the new lsx_parseposition function for the pad effect and update
the documentation accordingly.
Apply new timespec syntax to documentation example

Update the "acpo" example script shown in the man page to take
advantage of the syntax features that were added recently (chained
time specifications, positions). All calculations that previously had
to be done in the shell can now be done in SoX directly, and values
like "1:03" instead of only sample numbers are allowed as parameters
to the script.
Merge branch 'positions'

Extended syntax for specifying audio positions to several effects
(bend, delay, fade, pad, spectrogram, splice; trim had it before, but
now uses the shared lsx_parseposition function).