Dynamically load .so modules to expand functionality

Dynamically load .so modules to expand functionality

o Dynamically load all the .so files found in /libexec/nvmecontrol and
  /usr/local/libexec/nvmecontrol.
o Link nvmecontrol -rdynamic so that its symbols are visible to the
  libraries we load.
o Create concatinated linker sets that we dynamically expand.
o Add the linked-in top and logpage linker sets to the mirrors for them
  and add those sets to the mirrors when we load a new .so.
o Add some macros to help hide the names of the linker sets.
o Update the man page.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D18455

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