Add configuration support for setting the fetch timeout

This adds the configuration variable "FETCH_TIMEOUT", which
sets the timeout used by the fetch library to something the
user wants, retaining the old value of 30 as a default.
Clean up some error messages.

pkg_config_string() and pkg_config_bool() both emitted error
messages about pkg_config_int64. Fix this so they refer to
the correct functions.
Put FETCH_TIMEOUT at the end.

Perhaps I should have put them at the end, because of when they
were added rather than trying to be clever and grouping them
with similar items.
Merge pull request #461 from dannomac/clean-error

Clean up some error messages.
Merge pull request #460 from dannomac/master

Add fetch timeout, same as on release-1.0 branch.
Add pkg_try_installed()

Like pkg_is_installed(), but "returns" the package if any is found.

The need comes from pkg_is_installed() not able to "return" in any way
the package found. pkg_try_installed() extends pkg_is_installed()
making possible to test and query to package in one function call.
pkg_is_installed() is a now simple wrapper around pkg_try_installed().
Fix panic triggered when pkg add'ing a package already installed.

The `pkg_inst' pointer in pkg_add() was initialized to NULL on
declaration and never changed, triggering an assert in pkg_get2().
Merge branch 'master' of github.com:kAworu/pkgng