Minor fix for finding libcurl/building sslstrip

- Only look for CURL if we can build plugins (HAVE_PLUGINS).

- Use find_package to locate CURL headers *and* libs. This makes it
  possible to specify alternate CURL installation root via
  CMAKE_PREFIX_PATH. (ex: -DCMAKE_PREFIX_PATH=/usr/local/opt/curl/)

- Have cmake only build sslstrip if requirements for CURL and PCRE are
  met.
Cleaned up various compiler warnings generated by Wall, Wextra, Wshadow, Wlogical-op, Wcast-qual, and Wdisabled-optimization
Ensuring that proto gets null terminated.
Extra 0 make if statement always false.
Fixed uninitialized value (pcap file size) due to fstat failure
Fixed various invalid reads due to trusting packet data.
Added length checks so that ldap does not exceed the end of the packet
Poll's return value was never getting checked.
Moved the length check in get_dhcp_option
Stop an overlapped memcpy from occuring.
Merge pull request #92 from jacob-baines/master

GCC Cleanup
Merge pull request #94 from jacob-baines/offone

Extra 0 make if statement always false.
Merge pull request #96 from jacob-baines/pcapfile

Fixed uninitialized value (pcap file size) due to fstat failure
Merge pull request #97 from jacob-baines/snmp

Various Invalid Reads in SNMP
Merge pull request #98 from jacob-baines/ldap

Invalid Reads in LDAP
Merge pull request #99 from jacob-baines/poll

Poll's return value was never getting checked.
Merge pull request #100 from jacob-baines/dhcp1

Invalid Read In DHCP
Merge pull request #101 from jacob-baines/decoder_overlap

Del_decoder Overlapping Source and Destination
Merge pull request #90 from justfalter/curl_sslstrip_building

Minor fix for finding libcurl/building sslstrip
Fix for issue #102 (reverted part of pull request #92, sorry jacob-baines feel free to push again if you find a better approach)