Add pvaccess support to IOC stats

[simon-ess]

Closes #57 .

[anjohnson]

ifdef BASE_7_0 isn't a good way to detect whether PVA has been included. There are many different ways to build EPICS with PVA, it is possible to build it with 3.15 and maybe still with 3.14.12.5, although I wouldn't recommend that. Sites can also disable building the PVA modules in Base 7.0.x by editing modules/Makefile — unusual, but legitimate IMHO.

I would try using one of the variables defined in the installed cfg/CONFIG_PVACCESS_*, cfg/CONFIG_PVXS_* or cfg/CONFIG_QSRV* files.

[mdavidsaver]

Does handling of the environment variables really benefit from being build-time conditional? Why not just add these to epicsEnvVars.db? Which would also avoid some annoyance to dependent modules caused by renaming an installed .db file.

[anjohnson]

Not required for these particular environment variable changes, but note the new dbServerStats() routine I just added in epics-base/epics-base#592 which will give access to the RSRV and QSRV2 client and channel counts.