Now there can be only one log file instead of several which
logically work as a single file.

Possible names of redo log files: ib_logfile0,
ib_logfile101 (for just created one)

innodb_log_fiels_in_group: value of this variable is not used
by InnoDB. Possible values are still 1..100, to not break upgrade

LOG_FILE_NAME: add constant of value "ib_logfile0"
LOG_FILE_NAME_PREFIX: add constant of value "ib_logfile"

get_log_file_path(): convenience function that returns full
path of a redo log file

SRV_N_LOG_FILES_MAX: removed

srv_n_log_files: we can't remove this for compatibility reasons,
but now server doesn't use this variable

log_sys_t::file::fd: now just one, not std::vector

log_sys_t::log_capacity: removed word 'group'

find_and_check_log_file(): part of logic from huge srv_start()
moved here

recv_sys_t::files: file descriptors of redo log files.
There can be several of those in case we're upgrading
from older MariaDB version.

recv_sys_t::remove_extra_log_files: whether to remove
ib_logfile{1,2,3...} after successfull upgrade.

recv_sys_t::read(): open if needed and read from one
of several log files

recv_sys_t::files_size(): open if needed and return files count

redo_file_sizes_are_correct(): check that redo log files
sizes are equal. Just to log an error for a user.
Corresponding check was moved from srv0start.cc

namespace deprecated: put all deprecated variables here to
prevent usage of it by us, developers