TBR: i#511 drreg: simplify usage by combining multiple inits

Commit log for first patchset:
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i#511 drreg: simplify usage by combining multiple inits

Adds support for calling drreg_init() multiple times and combining the
requests.

Adds a new drreg option field, do_not_sum_slots, to support taking the
maximum rather than summing, to support libraries requesting a minimum
number of slots yet not adding to the simultaneous slots needed if the
client is already using drreg directly.

This simplifies the opcodes.c and stats.c clients, who no longer need to
initialize drreg at all.

This also simplifies the modxfer client, who can now not ask for slots.
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[bruening, 2017-01-30 18:23:28]

Committed as
https://github.com/DynamoRIO/dynamorio/commit/583b2fc994169461d29dd4cc720f85d...

Final commit log: 
---------------
i#511 drreg: simplify usage by combining multiple inits

Adds support for calling drreg_init() multiple times and combining the
requests.

Adds a new drreg option field, do_not_sum_slots, to support taking the
maximum rather than summing, to support libraries requesting a minimum
number of slots yet not adding to the simultaneous slots needed if the
client is already using drreg directly.

This simplifies the opcodes.c and stats.c clients, who no longer need to
initialize drreg at all.

This also simplifies the modxfer client, who can now not ask for slots.

Review-URL: https://codereview.appspot.com/313450043
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