linker: Enable debug logging via properties

This patch lets developers tune logging of dlopen/dlerror with
setting system property.

Note that for security purposes this option is disabled on user build
for non-debuggable apps.

For starters there are 3 debug options:
 dlerror - enables logging of all dlerrors
 dlopen - traces dlopen calls

To enable system-wide logging (works only for userdebug/eng builds)
use debug.ld.all property.

To enable logging for particular app use debug.ld.app.<appname> property.

Example: Running "adb shell setprop debug.ld.all dlerror,dlopen" will log all
dlerror message as well as trace all calls to dlopen.

Bug: http://b/29458203
Change-Id: I2392c80a795509e16fe5689d0500d18b99772a64