Change underline prefixes in symbols

Don't use underline prefixes, these are reserved for system use.

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Minor adjustments in p50/p60 common code

Cosmetic changes in comments and remove non-existent file from list
of files to be included in the distribution package.

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Fix instrument name size

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Fix merge conflict in theplayer.c

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Add instrument extras creation to extras API

There are three types of extra data: in module, in player channel and in
module instrument. Uniformize the way instrument extras are created and
start an "extras API" to also create the other types of extra data.

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Implement module extras

Allow formats to add arbitrary data to module headers using the
"extra" field.

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Set groundword for implementing channel extras

Implementing channel extras isn't as straightforward as other extras
because we reset values during replay.

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Implement channel extras

Move MED and His Master's Noise specific data out of the channel structure
and use them only if the corresponding module format is playing.

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>