Second alpha version in 3x branch

Removals

• Removed psync_rendir macro.
• Removed psync_file_rename function.
• Removed requirements.txt file from the project. It was only needed to
  install conan at CI phase.
• Drop Windows support. In fact, this project never built natively on Windows.
  All users could do is to build it using Cygwin or Msys. However, this project
  depends on some components (for ex. FUSE, POSIX threads and UNIX sockets) that
  either don't work under Windows, or have poor support, or require significant
  support costs. Given the fact that this project is intended for UNIX/Linux users,
  Windows support becomes an unaffordable luxury. Anyway, in the WSL/Docker era,
  things get a little easier, even for Windows users.
• Removed no longer used PSYNC_THREAD macro in favor of __thread.
• Removed no longer used PSYNC_DEFAULT_WINDOWS_DBNAME macro.
• Removed no longer used PSYNC_DEFAULT_WINDOWS_DIR macro.
• Removed no longer used psync_32to64 macro.
• Removed no longer used psync_bool_to_zero macro.
• Removed no longer used P_PRI_D64 macro.
• Removed no longer used psync_stat_ctime macro.
• Removed no longer used psync_inode_supported macro.
• Removed no longer used psync_sock_err_t typedef.
• Removed no longer used PSYNC_DIRECTORY_SEPARATOR macro.
• Removed no longer used PSYNC_DIRECTORY_SEPARATORC macro.
• Removed no longer used P_SEEK_CUR macro.
• Removed no longer used P_SEEK_END macro.

Features

• Added -dumpversion program option alias for --dumpversion.
• Amended tests.
• Provided launchd integration for macOS.
• Provided support to build project under arm64/aarch64 architectures.

Bugfix

• #87: Define _FILE_OFFSET_BITS to 64 on arm64 architectures.
  For more info about this macro refer to
  https://www.gnu.org/software/libc/manual/html_node/Feature-Test-Macros.html.

Changes

• Renamed overlay_callback pointer to function to poverlay_callback.
• Renamed overlay_message_t overlay data type to poverlay_message_t.
• Restructured the layout of CLI application.
• Bumped minimal C++ language version from C++11 to C++14.
• psync_check_result() now is macro, not function.
• Created a dedicated namespace for compat functions.
  Now they will all be in pcloudcc/compat/*.h
  • pcloudcc/compat/string.h: Some extra string manipulation functions
    not present in all standard C runtimes.
  • pcloudcc/compat/compiler.h: Compilers compatibility constants