fix ctypes.util.find_library usage

[mstone232]

Per https://docs.python.org/2/library/ctypes.html the find_library
function should be called "without any prefix like lib, suffix like .so,
.dylib or version number".

• change _LIBRARY_NAME from "libarchive" to "archive" as required

• move find_library logic after the *_LIBRARY_PATH logic to preserve
  current behavior. I suspect that the LD_LIBRARY_PATH logic is not
  actually necessary, but a side effect of this bug. I'm leaving it
  alone because I can't test all existing configurations to confirm
  that.

• if find_library returns a result, use it unconditionally. Trying to
  confirm it is unnecessary (find_library would have returned None
  otherwise) and impossible without duplicating all of the linker's
  logic. The result will be something like "libarchive.so.13" (without
  a path) which will just do the right thing when used with LoadLibrary.

• remove note about installing libarchive-dev, that isn't needed

[coveralls]

Pull Request Test Coverage Report for Build 16

• 4 of 4 (100.0%) changed or added relevant lines in 1 file are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.1%) to 70.37%

---

Files with Coverage Reduction	New Missed Lines	%
libarchive/library.py	2	72.41%

Totals
Change from base Build 14:	-0.1%
Covered Lines:	665
Relevant Lines:	945

---

💛 - Coveralls

[jocelynj]

I'm seeing an issue with python 3.9 that should be fixed by this commit:

    error: [Errno 2] No such file or directory: b'liblibarchive.a'

Is it possible to update this PR, and merge it to master?